Add SGV minutes-ago indicator

[mddub]

Add a configurable pie chart-like element to indicate how old the latest SGV reading is.

This is an insanely noisy PR because:

• every layout preset has properties for minutes-ago location, style, and colors
• virtually every screenshot test "gold" image has been updated with a recency indicator (and new dimensions for some layouts)
• the location of the "connection status" component is now configurable as well
• every integer now has an appropriate width, to make the app fit in the limited Aplite memory
• likewise, the GraphData struct holding SGV history has been removed in favor of directly accessing last_app_message()
• connection status component indicates when AppMessage can't be opened because the app is out of memory
• the README has been updated with new screenshots and more relevant instructions